The persistence of HIV-1 infection is the result of many factors, including rapid viral evolution to evade immunity, the establishment of reservoirs of both latent and cryptically replicating virus, and damage to the immune system caused directly or indirectly by virus replication. Our understanding of each of these factors remains inadequate to fully explain HIV-1 persistence and pathogenesis. The studies proposed will describe a previously unknown pathway of HIV-1 replication we have discovered that likely contributes to HIV-1 adaptation, to the establishment of viral reservoirs, and to pathogenesis. As a retrovirus, integration of HIV-1 DNA into the cellular chromosome is necessary for productive infection. Interestingly, 90-99% of HIV-1 DNA in vivo and in vitro remains unintegrated and by itself is unable to generate sufficient RNA and proteins to make new virions. However, our published studies reveal that in a productively infected cell, uDNA is complemented by the integrated provirus and completes its replication cycle. In other words, uDNA contributes to the replicating virus population and magnifies the amount of multiple infection. The result is an increased effective virus population size, abundant interactions among potentially divergent viruses, and enhanced virus evolution through recombination and mutation. This novel mechanism for HIV-1 replication is distinct from pre-integration latency and does not depend on subsequent integration by the uDNA. New preliminary data indicate the HIV-1 can superinfect cells and bypass integration, resulting in accelerated viral replication, an important parameter of viral fitness. We hypothesize that uDNA exerts a strong influence on viral evolution, persistence and pathogenesis. Owing to the high stability of circular forms of uDNA in non-proliferating cells, we hypothesize that uDNA constitutes a long-lived reservoir of virus whose replication is restored by productive reinfection of the host cell. Through a combination of experimental and analytical approaches, the following specific aims will test these hypotheses. Aim 1. What is the contribution of uDNA to the replicating virus population? Aim 2. Test the hypothesis that uDNA can function as a reservoir of latent viruses in T cells and macrophages. Aim 3. Examine the influence of uDNA on HIV-1 replication kinetics. Aim 4. Develop descriptive and predictive mathematical models of uDNA's contribution to HIV-1 replication and diversification to explore our main hypothesis.